The organic silicon polymer having silalkylenesiloxane units represented by formula: ##STR2## wherein R' represents a monovalent hydrocarbon group, and a represents a positive integer, has a higher resistance to chain scission by acid, alkali, and other ionic substances than does a diorganopolysiloxane. Consequently, in use in special fields has been studied. For example, it has been proposed that the organic silicon copolymer made of silalkylenesiloxane units and trifluoropropylmethylsiloxane units be used as the base polymer for a high-strength silicone rubber composition (Japanese Kokai Patent Application No. Hei 4-89866). As a method for manufacturing this type of organic silicon polymer having silalkylenesiloxane units, it is proposed that cleavage polymerization be performed for the cyclic silalkylenesiloxane (Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, 739-744, April, 1971, Japanese Kokai Patent Application Nos. 4-65428 and Hei 4-65429).
However, for the organic silicon copolymer made of silalkylenesiloxane units and trifluoropropylmethylsiloxane units disclosed in Japanese Kokai Patent Application No. Hei 4-89866, there is no regularity in the bonds of these siloxane units, and the fluorine content is small. Consequently, it is inappropriate for applications that require good surface lubrication property, surface modification property, and oil resistance. For the manufacturing method of the organic silicon polymer disclosed in Izvestiya Akademii Nauk SSSR, Japanese Kokai Patent Application Nos. Hei 4-65428 and Hei 4-65429, it is necessary to prepare 2,2,6,6-tetramethyl-1-oxa-2,6-disilacyclohexane, or other cyclic silalkylenesiloxane beforehand. Usually, however, it is difficult to prepare the cyclic silalkylenesiloxane. This is a disadvantage. Also, as reported in Izvcstiya Akademii Nauk in this method a portion of the polysilalkylenesiloxane is depolymerized and the yield of the organic silicon polymer yield decreases significantly. This is also a disadvantage.